Optical media having a high data density are ideal for storing large amounts of information, such as reference data bases, newspapers and books, library files, etc. Some of these media are valuable and a potential target for counterfeiting. It is believed, that in the future, the counterfeiting of optical media will be a significant problem. Currently, some optical media are protected in a limited way by software or by electronic devices. In principle, security devices such as optical thin films, holograms, gratings and micro-prisms can be attached to optical media to protect them. However, in practice, these attached security devices cannot be placed on the read side of the optical medium without affecting its normal operation. Therefore, it would be desirable for the media to have a security feature on the read side of the medium which will not interfere with the normal operation of the media, making it more difficult for counterfeiters to copy.
Optical media are used for storing a variety of information such as the storing of music, video images, text, computer data and so forth in the form of digital data. Data can be stored on diskettes, passports, credit cards, bank cards and other articles. Currently, optically-variable security devices, such as thin films, holograms, gratings, micro-prisms, are commonly used to protect some of these articles on the non-read side of the optical media; their optical features can be easily recognized by the public at large. Optically variable thin film security devices exhibiting a color change with a shift in viewing angle have been disclosed in U.S. Pat. No. 3,858,977 issued in January of 1975 in the name of Baird et at. Such devices are successfully used on large denominations of Canadian bank notes and on drivers' licenses in some jurisdictions. To enhance the security of these devices, U.S. Pat. No. 5,009,486 issued Apr. 23 1991 in the name of Dobrowolski et at. describes a form depicting, optical interference authenticating device. Dobrowolski et al. teach the introduction of additional patterns in thin film security devices by using masks during their deposition or by infrared laser buming to remove one or more layers of the thin film system. However, none of the above security devices taught by these patents can be applied to the read side of optical media where they would be most effective.
Currently, compact discs, CD ROMs and other optical media are protected to a very limited extent by indicia such as logos that are applied to the non-read side of the disk or media. Lately, counterfeit copies of legitimate commercial software have been offered for sale; many of these copies include logos on the non-read side of the disk that fool many consumers into believing they are purchasing a legitimate product. As time passes, and the sophistication of counterfeiters increases, the illegitimate products they attempt to market are more difficult to discern from the legitimate ones.